(1) Field of the Invention
This invention relates to the treatment of water-containing coal and, more particularly, to a process for the treatment of water-containing coal in which the water-containing coal is treated with a hydrocarbon oil to prepare the mixture consisting of the dehydrated coal and the hydrocarbon oil.
(2) Description of the Prior Art
Recently, the liquefaction of coal has come to attract much attention because it can offer a solution to oil shortage, and this subject is being studied more and more extensively. Typical methods for the liquefaction of coal includes the solvolysis process using a heavy oil obtained from petroleum and the hydrogenation process using a circulating solvent. The raw material used in the solvolysis process is a slurry of dehydrated coal in asphalt and that used in the hydrogenation process is a slurry of dehydrated coal in a circulating solvent.
Before being formed into a slurry, coal to be used in such coal liquefaction processes should desirably be freed of ash and moisture as completely as possible. In fact, some types of coals (i.e., bituminous coal and sub-bituminous coal) are subjected to a coal-preparation process for removing ash as completely as possible, because its removal after liquefaction is difficult. The use of water is effective in coal deashing process, and the separation efficiency becomes higher as the particle size of the coal is decreased. Accordingly, the current trend of the coal-preparation art is toward the treatment of finely-ground coal in a wet process. However, the use of water in coal deashing naturally causes the coal to be wetted with water, and the amount of water attached thereto increases as the particle size of the coal is decreased. Especially when the removal of ash is difficult, it is desirable to grind the coal to 1-mm size or finer prior to coal deashing process. In this case, the amount of water attached to the dressed coal can hardly be reduced to less than 20% by resorting to conventional mechanical means of dehydration. Moreover, when the coal processing plant is distant from the coal mine, economic advantages are obtained by crushing the coal to 4-mm size or finer and conveying it in the form of an aqueous slurry having a solids concentration of the order of 50%. In either case, the dehydration of such water-containing coal is a critical problem if it is to be used as a raw material for the liquefaction of coal.
On the other hand, so-called low-grade coals such as grass peat, peat, lignite, brown coal, some types of sub-bituminous coal, etc. are found in abundance all over the world. Not a few of them are low in ash content, capable of being mined at slight cost, and easy of liquefaction by the hydrogenation process. However, since these low-grade coals have strongly hydrophilic properties, they have a water content of at least 25% and, moreover, cannot be subjected to any coal-dressing process involving the use of water. Accordingly, the dehydration of these low-grade coals is disadvantageous from the viewpoints of technology, safety, and economics, and they are used exclusively for field power generation under the existing circumstances.
Thus, both high-grade and low-grade coals come to have a high water content and, therefore, require dehydration prior to their use in coal liquefaction processes. In carrying out this dehydration, it would be most desirable to employ a process which produces a mixture consisting of the dehydrated coal and a solvent for use in the liquefaction of coal and having an optimum coal concentration.